The present invention relates to optical devices and, in particular, it concerns hetero-modal interferometer optical sensors using spectral interrogation.
Optical evanescent-wave sensors in various forms have been exploited for highly sensitive chemical-biological sensing. Of these sensors, optical interferometric sensors utilize differential phase based detection for high sensitivity sensing. These sensors require a reference wave to mix with the signal in order to translate phase changes into detectable intensity changes. The reference can be provided in different ways with the most common being the dual-arm system used in the Mach-Zehnder configuration. Single channel multi mode waveguide interferometric sensors have also been reported, where the two paths pass along the same waveguide but in different propagation modes. This approach allows phase sensing without requiring a reference leg, and thus offers high sensitivity while maintaining the compactness and simplicity of a single channel waveguide. The use of modal interferometry was also reported in fiber sensors using circular, elliptical and photonic crystal multimode fibers with two or more propagating modes.
Different detection methods for optical sensors have been reported including intensity, phase, coupling angle, and spectral interrogation. Of these methods, spectral interrogation sensors use a broad bandwidth light source or a tunable laser, and detect variations in the sensed material by measuring changes in the wavelength dependence of the transfer power function induced by the sensing process. This method is widely used with many optical sensors configurations such as prism coupler-based surface plasmon resonance (SPR) sensors (Kretschmann configurations), grating-based sensors, waveguide SPR sensors, fiber sensors, and unbalanced Mach-Zehnder interferometer (MZI) based sensors. FIGS. 1A and 1B show examples the wavelength shift using spectral interrogation in conventional sensors: an SPR sensor (FIG. 1A) and unbalanced MZI sensor (FIG. 1B).
The sensitivity of sensing by spectral interrogation is determined by the wavelength shift Δλ of features in the transfer power function corresponding to a change in the measured parameter. It would therefore be advantageous to provide sensors and corresponding methods of sensing with enhanced wavelength shift Δλ of features in the transfer power function, thereby enhancing sensitivity of the sensor.